1. Field of the Invention
The present invention relates to an ultrasonic probe, more specifically, a broad-banded ultrasonic probe capable of transmitting and receiving ultrasonic waves having a plurality of frequencies.
2. Description of the Prior Art
Ultrasonic diagnoses have been extensively popularized as image diagnostics of high simplicity, safetiness, and economy and have been spreading the range of the examining subject in almost all the realm of the living body. Especially in the examination of the living body, however, different frequencies must be used depending on subjects to be examined. In the prior art, since the available frequencies are specific to respective ultrasonic probes, multiple kinds of ultrasonic probes are generally required for respective subjects. In the examination of the living body, for example, probes having a high frequency, e.g. 5-10 MHz, for examining the shallow regions and ones having a low frequency, e.g. 3.5-5 MHz, for examining the deeper regions. As stated above, it has been an inconvenience that probes having different frequencies have to be selected for use depending on subjects to be examined. Consequently, a broad-banded ultrasonic device using a single probe capable of transmitting and receiving various frequencies from low frequencies to high frequencies is now strongly called for.
Up to the present, several types of ultrasonic probes capable of transmitting and receiving a plurality of frequencies have been invented. For example, there are a type laminated with piezoelectric transducers each having different resonant frequency as taught in the Japanese patent laid-open publication Nos. 73861/1983, 172600/1988, and 173954/1988, a type devised with acoustic matching layers as disclosed in the Japanese patent laid-open publication No. 255044/1988, and a type comprising piezoelectric transducers having different resonant frequencies and alternately arranged as shown and described in the Japanese patent laid-open publication No. 68000/1987.
Any of those types has a defect, such as a complicated structure causing difficulties in manufacture or a narrow band for frequency response. The laminated type of ultrasonic probe, for example, requires to have a structure laminated with as many piezoelectric transducers as the number of different frequencies, causing complexity in manufacture and less economy. Also, with respect to the characteristics, since the laminated type has a structure with piezoelectric transducers having different resonant frequency laminated toward the direction of ultrasonic waves transmitted and received by the probe, the piezoelectric transducers act upon each other to interfere with the ultrasonic wave propagation when the probe transmits and receives ultrasonic waves, resulting in difficulty of obtaining acceptable results.
Further, in the type devised with acoustic matching layers, as the band cannot be widened more than that of the piezoelectric transducer, it is difficult to obtain satisfactory characteristics.
Still further, the type with alternately arrayed piezoelectric transducers having different resonant frequencies can be used in the form of an array type of ultrasonic probe, through the density in array of transducers having the same frequencies is low. Therefore, it is difficult to satisfy the most important requirements, for the array type probe, that the array density of transducers be high and an ultrasonic sound field capable of transmitting and receiving ultrasonic waves having high directivity with the grating lobe suppressed as much as possible be formed, resulting in degradation of the characteristics.
In addition, as a type different from those described above, in the Japanese patent laid-open publication No. 22040/1983 there is proposed an array type of probe in which arranged are the piezoelectric transducers which are continuously different in thickness in the direction perpendicular to the scanning direction to cause the resonant frequencies to continuously differ from each other in that direction. This system, however, also has a great difficulty in manufacture of the above-mentioned piezoelectric transducers and is hard to be put to practical use.